1. Field of the Invention
The invention relates to a communication apparatus such as a facsimile apparatus or the like which is connected to an ISDN.
2. Related Background Art
A facsimile apparatus is widespread in the society and is an apparatus which is indispensable as a business tool. On the other hand, in the communication line, the services of the ISDN (Integrated Service Digital Network) are just beginning, the integration of the communication is progressing, and the facsimile apparatus is expected as to be an infrastructure of the next generation of communication apparatus.
In the ISDN, it is possible to inter-network connect with the conventional PSTN (Public Service Telephone Network). Therefore, most of the ISDN facsimile apparatuses which are connected to the ISDN have the ability to communicate with both of the G4 facsimile apparatus which is connected to the ISDN and the G3 facsimile apparatus which is connected to the PSTN. Consequently, in the ISDN facsimile apparatus, the number of necessary parts such as ISDN interface, MODEM for the G3 facsimile apparatus, PCM-CODEC, HDLC controller, and the like is much greater than that of the G3 facsimile apparatus. Thus, there are problems such that a scale and cost of the hardware are much greater than those of the G3 facsimile apparatus. It is, accordingly, a present situation that the cost of the apparatus is high and, in spite of the fact that there are many advantages of the ISDN line such as high-speed communication, multiaccess, and the like, the G4 facsimile apparatus is not used in place of the G3 facsimile apparatus.
In a telephone using the PSTN, even in case of a power failure, the dialing function, ringing function, and off-hook detecting function are made operative by merely supplying a power source from the network and the telephone can be used. Therefore, even in case of the power failure, emergency information or the like can be also transmitted. In a communication terminal using the ISDN, particularly, in the terminal having the facsimile function, however, in order to make the telephone operative, a number of circuits such as control unit, communication control unit, and the like must be made operative in terms of the construction and an electric power consumption of the circuit cannot be satisfied merely from the power supply of the network. Even when a backup power source is used, the power source can be held for only a short time. The cost is also high. In case of a power failure, consequently, the telephone attached to the ISDN terminal cannot be used.